Some electric motors are operated at low speeds at which there is an essentially periodic fluctuation, or ripple, in the output torque. Such ripple, one form of which is called "cogging," is primarily due to factors such as unbalanced and/or coarse resolution driving phases or limited numbers of poles on the rotor and/or stator. These kinds of factors give rise to periodic variations in the magnetic field strength in the air gap between cooperating rotor and stator poles as the motor turns and hence to the ripple effect. Torque ripple can also occur as a result of, e.g., irregularities in motor bearings which impose corresponding irregularities in the loading of the motor. Torque ripple is necessarily accompanied by corresponding motor speed variations which also are essentially periodic and at frequencies higher than the motor speed in revolutions per minute (RPM). If the motor is driving, e.g., a coating drum onto which a fluid is being sprayed, the speed fluctuations result in a deposited film which has an undesirable nonuniform thickness.
Electric motor output torque ripple is one of the factors that concerns electric motor system designers. Several aspects of the problem are described in "Drive Scheme Holds Key To DC Motor Performance" by M. B. McCormick, EDN, Jan. 19, 1989. It is said that a motor with sinusoidal commutation has nearly zero torque ripple over a complete revolution of the motor. However, in some applications significant troublesome ripple still remains.
Similarly, in "A Brushless Motor Evaluation" by B. Bessenyei and T. Niertit, Motion, July/August 1988, pages 3, 4, 6-8, and 10, solutions such as balancing the motor currents and adding a high-inertia load are considered. However, significant ripple still remains in some applications. [Note that, due to a typographical error, the figure numbers for FIGS. 5 and 6 in this paper were inadvertently interchanged.]
Flywheels have long been used to smooth torque ripple in the output of various kinds of motors, including electrical motors; but flywheels are bulky, and inconvenient to use for motors operated at speeds below say 100 RPM. Efforts also have been made to combat the torque ripple problem by specific motor structure design techniques, and one example is shown in the U.S. Pat. No. 4,947,066 to B. A. Ghibu et al.
There are many motor control servo systems that use position signals derived from a motor output to control motor speed. In these, any given speed change and correction may take place over the course of multiple revolutions of the motor armature. Two examples are shown in the U.S. Pat. Nos. 4,885,793 to J. Tabuchi and 4,259,698 to S. Takada. Such motor speed control servo systems are typically found in systems in which the motor speed is substantially greater than the recurrence rate of any speed changes of interest. Those systems usually have a relatively slow response which is unable to deal with an application in which there are recurrent speed variations during each revolution of the motor's armature as is the case where torque ripple is a problem.
A number of attempts have been made to reduce the impact of torque ripple by employing electronic circuit means. In U.S. Pat. No. 4,868,477 to F. J. Anderson et al., static drive current measurements are made at each of many angular positions of the motor shaft; and for each torque rating at which the motor is to be operated the current values are stored in read only memories (ROMs) for the respective motor drive current phases. Those values are then read out to provide drive current when the motor is being operated under normal rotational load. This is an awkward and time consuming process which provides compensation in the form of a composite of all torque effects which appear under static conditions. The U.S. Pat. No. 4,943,760 to J. V. Byrne et al. is another of the type that uses static tests to determine a compensating waveform for each phase of motor drive current.
U.S. Pat. No. 3,919,609 to H. Klautschek et al. shows the comparison of actual output torque measurements to a reference torque and use of the difference to correct drive current for reducing ripple. This too involves measurement of gross torque, and its real time style of operation puts a significant premium on dealing adequately with issues of circuit and hardware inertia and time constants. Another torque comparison and current control system is shown in U.S. Pat. No. 4,240,020 to T. Okuyama et al.
U.S. Pat. No. 4,890,048 to L. W. Hunter shows compensation for torque ripple due to motor drive current supply effects by employing an accumulator to dampen hunting caused by the input signal.